The shape of an end surface of a roller affects friction heat due to contact with a flange and edge loads near a chamfer of the roller. Thus, a portion of the roller end surface that affects such factors is ground to ensure necessary dimensional accuracy. For example, in order to reduce friction heat, the spherical portion of the roller end surface that is continuous with the chamfer is formed by grinding. A cup-shaped grinder having a cylindrical grinding portion is typically used for such grinding (JP Patent Publication 2005-297181 (see especially paragraphs 0002-0004, 0016-0018, and 0023, and FIGS. 3 and 4)).
FIGS. 6(a) and 6(b) show a conventional grinding method using a cup-shaped grinder, which is called a two-roll, one-shoe method. In this method, a workpiece 61 formed with a radially outer surface of a roller is held in a lateral position by two roll type driving rings 62, a shoe 63, and a packing plate 64 and rotated about its center axis. Simultaneously, a grinder 65 is cut into an end surface of the workpiece 61. When the grinder 65 is pressed against the end surface, the grinder 65 is inclined relative to the center axis of the workpiece 61 so that a spherical surface can be formed utilizing the self-sharpening characteristics of the grinder 65.
When grinding the workpiece by pressing the cup-shaped grinder while rotating the workpiece about its center axis in the above manner, as shown in FIGS. 7(a) and 7(b), the workpiece 61 or 71 may be inserted into a support surface 66a or 72a of a jig 66 or 72 in a lateral position, and the grinder 65 or 73 may be pressed against the workpiece while rotating the jig 66, 72 coaxially with and together with the workpiece 61, 71. Since the radially outer surface of the workpiece 61, 71 is supported by the support surface 66a, 72a of the jig 66, 72, the jig serves to increase the apparent outer diameter of the workpiece 61, 71, thus increasing positioning accuracy in the direction perpendicular to the center axis of the workpiece 61, 71. The radially outer surface of the workpiece 61, 71 is finished to a rolling surface of one of a tapered roller, a convex roller and a cylindrical roller. Since the radially outer surface of the workpiece 61, 71 is used as a reference surface when positioning the workpiece for grinding, it has to be finished to sufficient accuracy.
In another conventional method shown in FIGS. 8(a) and 8(b), an end surface of a workpiece 81 that is not machined is magnetically fixed to a spherical attracting surface 82a of an electromagnetic turntable 82. A grinder 83 which rotates about an axis parallel to the rotation axis of the electromagnetic turntable 82 is held in a fixed position. The grinder 83 is pressed against the workpiece 81 based on predetermined feed control, while revolving the workpiece 81 around the axis of the turntable 82 with the turntable.
In the conventional arrangement of FIGS. 6(a) and 6(b), if the angle of the radially outer surface of the workpiece 61 for a tapered roller is large, force acts on the workpiece 61 that tends to push out the workpiece 61, which makes it difficult to stably achieve sufficient machining accuracy. In the conventional arrangement of FIG. 7(a) too, if the angle of the radially outer surface of the workpiece 61 is large, since the wedge effect relative to the support surface 66a of the jig 66 is scarcely produced, it is difficult to stably achieve sufficient machining accuracy. In the conventional arrangement of FIG. 7(b), since a workpiece 71 for a convex roller is laterally inserted into the support surface 72a of the jig 72, the jig 72 can retain the workpiece 71 only up to its maximum outer diameter portion. Thus, workpieces that can be stably supported by the jig 72 are limited to workpieces having such an asymmetrical radially outer surface that the maximum outer diameter portion is located nearer to the ground portion as shown in FIG. 7(b). The conventional arrangement shown in FIG. 7(b) cannot be used for workpieces for any convex rollers.
In the conventional arrangement shown in FIGS. 8(a) and 8(b), since the workpiece 81 is fixed in position simply by magnetically attracting its end surface which is not ground to the electromagnetic turntable 82, it is difficult to uniformly retain the attitude of the workpieces 81. This deteriorates machining accuracy. Since the workpiece 81 is magnetized, it is necessary to demagnetize the workpiece after grinding. Further in the conventional arrangement shown in FIGS. 8(a) and 8(b), since the accuracy of the surface finished by grinding depends on the accuracy of the sphere of spherical attracting surface 82a of the electromagnetic turntable 82, a long time is needed when exchanging the electromagnetic turntable 82 for setup change, or when re-polishing the electromagnetic turntable 82 for quality adjustment. Productivity is thus extremely low.
One way to improve productivity of the grinding operation would be to replace the grinding operation with a cutting operation. Ordinarily, tapered rollers and convex rollers are formed by hardening steel, typically bearing steel. In the retaining methods of FIGS. 6(a)-8(b), it is difficult to achieve retaining force sufficient to allow cutting of a workpiece made of hardened steel.